To calculate combined uncertainties use absolute or relative uncertainties depending on the combination of variables which give a final value x:
If
• x = a + b or x = a - b :- then use ux = √{(ua)2 + (ub)2}

• x = a×b or x = a/b :- then use Rux = √{(Rua)2 + (Rub)2}

• x = k×a (where k is a constant) :- then use Rux = Rua or ux = k×ua

• x = an (where n is a constant) :- then use Rux = n×(Rua)

Note that it is possible to use the simple (not percentage) relative uncertainties (i.e. without introducing the ‘100’ into the calculations), but it is necessary to be consistent throughout the calculation. We use percentage uncertainty here because many scientists are more familiar with expressing relative uncertainty as a percentage.

See Study Text: Section 8.3.3

Absolute and relative uncertainty

The uncertainty of ±0.03 cm3 in a 10 cm3 class A graduated pipette would be considered as the absolute uncertainty.

Absolute uncertainties have the same units as the value itself (e.g. cm3 in the above example), relative uncertainties are a simple ratio with no units, and percentage uncertainties are ratios expressed as percentages.